1. Field of the Invention
The present invention relates generally to vacuum transport belt, apparatus, such as useful in ink-jet hard copy apparatus and methods of operation and, even more specifically, to a restricted flow vacuum system providing media cockle control and having minimal airflow-induced ink drop trajectory effects.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in OUTPUT HARDCOPY [sic] DEVICES, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988). As providing background information, the foregoing documents are incorporated herein by reference. Further details of basic ink-jet printing technology are also set forth below in the Detailed Description of the present invention with respect to FIG. 1.
It is known to use a vacuum induced force to adhere a sheet of flexible material to a surface, for example, transporting sheet metal, holding a sheet of print media temporarily to a transport system or platen, and the like. Hereinafter, xe2x80x9cvacuum induced forcexe2x80x9d is also referred to as xe2x80x9cvacuum induced flow,xe2x80x9d xe2x80x9cvacuum flow,xe2x80x9d or more simply as just xe2x80x9cairflow,xe2x80x9d xe2x80x9cvacuumxe2x80x9d or xe2x80x9csuction,xe2x80x9d as best fits the context. Such vacuum holddown systems are a relatively common, economical technology to implement commercially and, in printing technology, can improve hard copy apparatus throughput specifications. For example, it is known to provide a rotating drum with holes through the surface wherein a vacuum type airflow through the chamber formed by the drum cylinder provides a suction force at the holes in the drum surface (see e.g., U.S. Pat. No. 4,237,466 for a PAPER TRANSPORT SYSTEM FOR AN INK JET PRINTER (Scranton) or U.S. Pat. No. 5,081,506 for a TRANSFER SYSTEM FOR A COLOR PRINTER (Borostyan)). The term xe2x80x9cdrumxe2x80x9d as used hereinafter is intended to be synonymous with any curvilinear implementation incorporating the present invention; while the term xe2x80x9cplatenxe2x80x9d can be defined as a flat holding surface, in hard copy technology it is also used for curvilinear surfaces, e.g., the ubiquitous typewriter rubber roller; thus, for the purposes of the present application, xe2x80x9cplatenxe2x80x9d is used generically for any shape paper holddown surfacexe2x80x94stationary or movablexe2x80x94as used in a hard copy apparatus. Permeable belts traversing a vacuum inducing support have been similarly employed (see e.g., Scranton and U.S. patent application Ser. No. 09/163,098 by Rasmussen et al. for a BELT DRIVEN MEDIA HANDLING SYSTEM WITH FEEDBACK CONTROL FOR IMPROVING MEDIA ADVANCE ACCURACY (assigned to the common assignee of the present invention and incorporated herein by reference)).
Generally in a hard copy apparatus implementation, the vacuum device is used either to support cut-sheet print media during transport to and from a printing station (also known as the xe2x80x9cprint zonexe2x80x9d or xe2x80x9cprinting zonexe2x80x9d) of a hard copy apparatus, to hold the sheet media at the printing station while images or alphanumeric text are formed, or both. In order to further simplify description of the technology and invention, the term xe2x80x9cpaperxe2x80x9d is used hereinafter to refer to all types of print media and the term xe2x80x9cprinterxe2x80x9d to refer to all types of hard copy apparatus; no limitation on the scope of the invention is intended nor should any be implied.
In essence, the ink-jet printing process involves digitized, dot-matrix manipulation of drops of ink, or other liquid colorant, ejected from a pen onto an adjacent paper. One or more ink-jet type writing instruments (also referred to in the art as an xe2x80x9cink-jet penxe2x80x9d or xe2x80x9cprint cartridgexe2x80x9d) include a printhead which generally consists of drop generator mechanisms and a number of columns of ink drop firing nozzles. Each column or selected subset of nozzles (referred to in the art as a xe2x80x9cprimitivexe2x80x9d) selectively fires ink droplets (typically each being only a few picoliters in liquid volume) that are used to create a predetermined print matrix of dots on the adjacently positioned paper as the pen is scanned across the media. A given nozzle of the printhead is used to address a given matrix column print position on the paper (referred to as a picture element, or xe2x80x9cpixel.xe2x80x9d). Horizontal positions, matrix pixel rows, on the paper are addressed by repeatedly firing a given nozzle at matrix row print positions as the pen is scanned. Thus, a single sweep scan of the pen across the paper can print a swath of dots. The paper is stepped to permit a series of contiguous swaths. Dot matrix manipulation is used to form alphanumeric characters, graphical images, and even photographic reproductions from the ink drops. Page-wide ink-jet printheads are also contemplated and are adaptable to the present invention.
A well-known phenomenon of wet-colorant printing is xe2x80x9cpaper cockle,xe2x80x9d the irregular surface produced in paper by the saturation and drying of ink deposits on the fibrous medium. As a sheet of paper gets saturated with ink, the paper grows and buckles in a seemingly random manner. Paper printed with images are more saturated with colorant than simple text pages and thus exhibit great paper cockle. Colors formed by mixing combinations of other color ink drops form greater localized saturation areas and also exhibit greater cockle tendencies.
As the ink-jet writing instrumentsxe2x80x94often scanning at a relatively high rate across the paper-expel minute droplets of ink onto adjacently positioned print media and sophisticated, computerized, dot matrix manipulation is used to render text and form graphic images, the flight trajectory of each drop is critical to print quality. Printing errors (also referred to in the art as xe2x80x9cartifactsxe2x80x9d) are induced or exacerbated by any airflow in the printing zone. Thus, use of a vacuum platen and vacuum transport device in the printing zone of an ink-jet printer creates an added difficulty for the system designer. One solution to the problem is set out in applicants"" pending application Ser. No. 09/514,830, filed on Feb. 28, 2000, for a LOW FLOW VACUUM PLATEN FOR AN INK-JET HARD COPY APPARATUS. In essence, it employs a platen having an array of vacuum ports that are each filtered. The filter is constructed to provide restricted airflow such that media holddown pressure remains substantially uniform when the platen is either fully covered or partially uncovered. The filter mechanism provides airflow restrictions such that ink drop flight trajectories in the printing zone are unaffected, acoustic dampening of the vacuum pump is provided, and vacuum pressure is kept relatively high at the print media edges.
There is still a need for a commercial, low-cost, vacuum system for use in an ink-jet printing zone which will assist in minimizing cockle and provide a minimal airflow impact on ink-jet drop flight trajectory.
In a basic aspect, the present invention provides a vacuum platen system for transporting a sheet material, comprising: a platen having ports permitting airflow therethrough at predetermined positions of a surface thereof; a vacuum device associated with the platen and inducing the airflow; and a transport belt superjacent the surface, having an array of belt perforations such that each perforation through the belt has a diameter substantially less than the diameter the ports.
In another basic aspect, the present invention provides a method for transporting print media across a vacuum platen associated with a vacuum inducing mechanism, comprising the steps of: drawing a vacuum through a plurality of vacuum ports distributed across the platen; and transporting ink-jet print media across the platen in a predetermined direction by a perforated belt associated with the platen so as to restrict flow by a combined construct comprising the platen and the belt.
In another basic aspect, the present invention provides an ink-jet hard copy apparatus comprising: an ink-jet writing instrument associated with a printing zone within the apparatus; an endless loop vacuum belt system for transporting print media to and from the printing zone; and a vacuum platen system located proximate the printing zone, the vacuum platen system having a platen, having a plurality of vacuum ports therethrough, a vacuum chamber, and a vacuum device for maintaining a negative pressure within the chamber such that an airflow is established through the vacuum ports into the chamber, wherein the vacuum belt system has a belt having perforations, each of said perforations being of a smaller size than each of said ports such that a uniform vacuum holding pressure is exerted on a sheet of print media carried by the belt across the platen and the airflow superjacent the belt in the printing zone is less than an airflow that affects ink drop flight trajectories.
In another basic aspect, the present invention provides a vacuum flow restricting print media transport apparatus comprising: a perforated belt and ported platen combination having an effective belt porosity less than platen porosity.
In another basic aspect, the present invention provides a method for controlling airflow in an ink-jet apparatus having a vacuum transport belt for transporting ink-jet media through a printing zone, comprising the steps of: suspending the vacuum belt across a vacuum source having essentially no physical support of the belt in the printing zone; and providing appropriate flow restriction in the printing zone by controlling the areal density of perforations in the belt based on specified design parameters and intended media usage.
In another basic aspect, the present invention provides for a method for controlling airflow in an ink-jet apparatus having a vacuum transport belt for transporting ink-jet media through a printing zone, comprising the steps of: suspending the vacuum belt across a vacuum source having essentially no physical support of the belt in the printing zone; and providing appropriate flow restriction in the printing zone by controlling the areal density of perforations in the belt based on specified design parameters and intended media usage.
Some of the advantages of the present invention are:
it provides a vacuum force sufficient for holding media in place against cockle deformation tendencies while being wetted by ink deposited thereon;
it provides a low flow vacuum system with minimal airflow induced ink drop directionality errors;
it provides a substantially uniform media holddown pressure when the platen is either fully covered or partially uncovered;
it provides a low flow platen that allows vacuum box pressure to remain relatively constant whether or not paper is fully covering the platen, thus compensating for different sized print media;
it allows for various media sizes and thicknesses to be held down with substantially the same pressure without requiring a large vacuum source;
it reduces acoustic levels caused by a vacuum induced airflow;
it provides a platen that is resistant to clogging by ink and paper dust;
it provides a belt that is available for cleaning off ink and paper dust;
it provides improved vacuum holding at paper edges;
it reduces platen construction complexity, resulting in less piece parts and lower manufacturing costs;
it eliminates vacuum leakage between ports;
it provides a media transport belt construct having better heat transfer characteristics;
it provides a media transport belt that is less subject to non-productive heat loss; and
it provides a more durable media transport belt.
The foregoing summary and list of advantages is not intended by the inventors to be an inclusive list of all the aspects, objects, advantages and features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01(d) merely to apprise the public, and more especially those interested in the particular art to which the invention relates, of the nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings.